Based on some experiments I did over Thanksiving, it appears that how soon you can fetch another fastpass depends on what ride the new Fastpass would be for as well as how busy the park is.
But the next-fetch time printed on the Fastpass you already have is the next-fetch time for the ride that Fastpass is for.
Jungle Cruise might, at that hour on that day, have had a shorter next-fetch time than, say, Space Mountain.
So if my guess is correct and you fetched a Fastpass for Space Mountain at 10 AM and the return (ride time) was 3 PM., the worst case next-fetch time of about T plus two hours is shown on that Fastpass. If you went to Jungle cruise just one hour later (11 AM) and if Jungle Cruise had a next fetch time of T plus one hour rather than T plus two hours, you will succeed in getting a Fastpass before the printed next fetch time of about noon on the Fastpass you already have.
Once I found the next-fetch time for Haunted Mansion to be T plus five minutes.
If there are system malfunctions, there may be other anomalies, usually in the guest's favor.
More Disney hints:
http://members.aol.com/ajaynejr/fastnext.htm